“RAID” stands for Redundant Array of Independent Disks and is a method of storing data on multiple hard disks. When disks are arranged in a RAID configuration, the computer sees them all as one large disk. However, they operate much more efficiently than a single hard drive. Since the data is spread out over multiple disks, the reading and writing operations can take place on multiple disks at once. This can speed up hard drive access time significantly.
To date, many different levels of RAID have been introduced. Of these RAID levels RAID 0, 1, 5, & 6 are common in industry. RAID 0 implements a performance oriented data striping technique. RAID 1 utilized mirrored storage devices. In other words, data was written identically to at least two disks. Thus, if one disk failed, the data could be retrieved from one of the other disks. Of course, a level 1 RAID system requires the cost of an additional disk without increasing overall capacity in exchange for decreased likelihood of data loss. RAID 5 introduced a parity protection scheme with that addition of one disk. Data and parity striping techniques are applied where data and parity information are distributed throughout the disk array. RAID 6 is an extension to RAID 5 supporting multiple simultaneous disk failures.
In today's RAID controllers, for example, a DMA (Direct Memory Access) engine performs a portion of a RAID operation while a SAS (Serial Attached SCSI (Small Computer System Interface)) core performs the remainder of the operation. A significant amount of CPU (Central Processing Unit) overhead is involved coordinating the DMA engine operations with the SAS core operations. This limits overall performance. The term “SAS core” refers to the firmware or hardware that implements the SAS.